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The Journal of Chemical Physics

Photomanipulation of external molecular modes: A time-dependent self-consistent-field approach

Zong-Chao Yan, Tamar Seideman

Published Sep 1, 1999• 37 citations
Publisher

Abstract

We introduce a time-dependent self-consistent-field approach for studying the coupled rotational–translational motion of molecules subject to the macroscopic potential induced by a spatially inhomogeneous, polarized laser field. A hybrid classical-quantum-mechanical variation of the scheme is employed to investigate the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis in a field-free region of space. A semiclassical-quantum-mechanical variation is used to describe the rotational–translational dynamics of cold molecules confined to an optical trap.

DOI
10.1063/1.479184

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We introduce a time-dependent self-consistent-field approach for studying the coupled rotational–translational motion of molecules subject to the macroscopic potential induced by a spatially inhomogeneous, polarized laser field. A hybrid classical-quantum-mechanical variation of the scheme is employed to investigate the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis in a field-free region of space. A semiclassical-quantum-mechanical variation is used to describe the rotational–translational dynamics of cold molecules confined to an optical trap.
We develop a general framework for describing the interaction of molecules with a nonperturbative intensity gradient and note several limits, including the two-level Hamiltonian of atom trapping and the quasistatic Hamiltonian of molecular optics. The theory is used to propose a new molecular optics scheme which complements previous work in the area of spatial manipulation of molecules in intense fields by introducing the possibility of reflecting molecules with light. Repulsive interaction obtains in the limit of small level spacings as compared to the field frequency, attained by exciting the molecules to high Rydberg states. Some of the potential applications of the repulsive interaction are illustrated and others are noted. We address the problem of molecular alignment in intense laser fields and describe the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis after exit from the laser focus, under field-free conditions.
Several laser techniques have been suggested and demonstrated recently for preparing polarizable molecules in rapidly spinning states with a disk-like angular distribution. We consider motion of these spinning disks in inhomogeneous fields and show that the molecular trajectories may be precisely controlled by the tilt of the plane of the laser-induced rotation. The feasibility of the scheme is illustrated by optical deflection of linear molecules twirled by two delayed cross-polarized laser pulses. These results open new ways for many applications involving molecular focusing, guiding, and trapping and may be suitable for separating molecular mixtures by optical and static fields.
The properties of molecule-optical elements such as lenses or prisms based on the interaction of molecules with optical fields depend in a crucial way on the molecular quantum state and its alignment created by the optical field. Herein, we consider the effects of state-dependent alignment in estimating the optical dipole force acting on the molecules and, to this end, introduce an effective polarizability which takes proper account of molecular alignment and is directly related to the alignment-dependent optical dipole force. We illustrate the significance of including molecular alignment in the optical dipole force by a trajectory study that compares previously used approximations with the present approach. The trajectory simulations were carried out for an ensemble of linear molecules subject to either propagating or standing-wave optical fields for a range of temperatures and laser intensities. The results demonstrate that the alignment-dependent effective polarizability can serve to provide correct estimates of the optical dipole force, on which a state-selection method applicable to nonpolar molecules could be based. We note that an analogous analysis of the forces acting on polar molecules subject to an inhomogeneous static electric field reveals a similarly strong dependence on molecular orientation.
This chapter describes a versatile technique to control molecular rotation and translation. The technique is based on the combined effect of external electric fields that act concurrently on the molecular permanent and induced dipole moments. This synergistic effect arises for any polar molecule, as only an anisotropic polarizability, along with a permanent dipole moment, is required. This is always available in polar molecules. The effect of the combined fields can be achieved either by superimposing an electrostatic and a far-off resonant optical field (i.e., one that supplies electric field strength but does not cause transitions) that act, respectively, on the molecule's permanent and induced electric dipole, or by making use of a unipolar electromagnetic pulse that seizes the permanent and induced dipole moments simultaneously. If the combined electric fields are homogeneous, only molecular rotation is affected, whereas inhomogeneous fields affect both molecular rotation and translation. As noted in the chapter, the ability to manipulate molecular rotation and translation has wide-ranging applications – in research areas as diverse as reaction dynamics, spectroscopy, higher harmonic generation and molecular orbital imaging, the focusing and trapping of molecules, as well as quantum simulation and computing.
Selection of state-insensitive or ``magic'' trapping conditions with ultracold atoms or molecules, where pairs of internal states experience identical trapping potentials, brings substantial benefits to precision measurements and quantum computing schemes. Working at such conditions could ensure that the detrimental effects of inevitable inhomogeneities across an ultracold sample are significantly reduced. However, this aspect of confinement remains unexplored for ultracold polar molecules. Here, we present means to control the ac Stark shift of rotational states of ultracold diatomic polar molecules, when subjected to both trapping laser light and an external electric field. We show that both the strength and relative orientation of the two fields influence the trapping potential. In particular, we predict ``magic electric field strengths'' and a ``magic angle,'' where the Stark shift is independent of the dc external field for certain rotational states of the molecule.
We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the “molecular lens” are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed.